1. Field of the Invention
The present invention relates to a driving apparatus and driving method for an electron source having a plurality of electron-emitting devices. The present invention further relates to a driving method for an image-forming apparatus using the electron source.
2. Description of the Related Art
Electron-emitting devices heretofore known are generally grouped into two types: a thermionic cathode and a cold cathode. The cold cathode includes field-emission (FE) devices, metal-insulator-metal (MIM) devices, and surface conduction electron-emitting devices.
For example, an FE-type device, such as the one disclosed by W. P. Dyke and W. W. Dolan in “Field Emission”, Advance in Electron Physics, 8,89 (1956), or the one disclosed by C. A. Spindt in “PHYSICAL Properties of thin-film field emission cathodes with molybdenum cones”, J. Apply. phys., 47, 5248 (1976), is known.
An MIM-type device, such as the one disclosed by C. A. Mead in “Operation of Tunnel-Emission Devices”, J. Appl. Phys. 32,646 (1961), is known.
Also, examples of devices which have been recently studied are as follows: Toshiaki. Kusunoki, “Fluctuation-free electron emission from non-formed metal-insulator-metal (MIM) cathodes Fabricated by low current Anodic oxidation”, Jpn. J. Appl. Phys. vol. 32 (1993) pp. L1695, and Mutsumi Suzuki et al., “An MIM-Cathode Array for Cathode luminescent Displays”, IDW'96, (1996) pp. 529.
As an example of the surface conduction electron-emitting device, there is known the one described in Elinson's report (M. I. Elinson, Radio. Eng. Electron Phys., 10 (1995)) or the like. The surface conduction electron-emitting device is a device utilizing the phenomenon in which a current is caused to flow in a small-area thin film formed on a substrate so as to be parallel to the film surface, so that a electron emission is realized. As the surface conduction electron-emitting device, there are reported a device using an SiO2 thin film described in the Elinson 's report, a device using an Au thin film (G. Dittmer. Thin Solid Films, 9,317 (1972)), a device using an In2O3/SnO2 thin film (M. Hartwell and C. G. Fonstad, IEEE Trans. ED Conf., 519 (1983)) and the like.
Various techniques may be adopted to arrange the electron-emitting devices. As one example, a plurality of electron-emitting devices are arranged in an X direction and a Y direction to form rows and columns. There may be used a passive matrix arrangement where once end of the electrode of each of the plural electron-emitting devices arranged on the same row is commonly connected to X-directional wiring, while the other end of the electrode of each of the plural electron-emitting devices arranged on the same column is commonly connected to Y-directional wiring. This passive matrix configuration will be described in detail below with reference to FIG. 12.
X-directional wiring 62 includes n lines (Dx1, Dx2, . . . , Dxm) and is constructed from a conductive metal or the like that has been formed using a vacuum deposition method, a printing method, a sputtering method, or the like. The material, thickness, and width of the wiring are determined as appropriate. Y-directional wiring 63 includes n lines (Dy1, Dy2, . . . , Dyn) and is produced in the same manner as the X-directional wiring 62. An interlayer insulating layer (not shown) is provided between the X-directional wiring 62 including the m lines and the Y-directional wiring 63 including the n lines so as to electrically separate these wirings (m and n are each a positive integer).
The interlayer insulating layer (not shown) is formed using SiO2 or the like with a vacuum deposition method, a printing method, a sputtering method, or the like. For instance, the interlayer insulating layer having a desired shape is formed to cover the entire or a part of the surface of a substrate 61 on which the X-directional wiring 62 has been formed. In particular, the thickness, material, and production method of the interlayer insulating layer are determined as appropriate so that the interlayer insulating layer is resistant to potential differences at intersections of the X-directional wiring 62 and the Y-directional wiring 63. The X-directional wiring 62 and the Y-directional wiring 63 are led out to the outside as external terminals.
There may be a case where the m lines of the X-directional wiring 62 constituting electron-emitting devices 64 double as cathode electrodes. Also, there may be a case where the n lines of the Y-directional wiring 63 double as gate electrodes. Further, there may be a case where the interlayer insulating layer doubles as an insulating layer between the gate electrodes and the cathode electrodes.
To select the rows of the electron-emitting devices 64 arranged in the X-direction, a scanning signal applying means for applying a scanning signal is connected to the X-directional wiring 62. On the other hand, to modulate each column of the electron-emitting devices 64 arranged in the Y-direction in accordance with an input signal, a modulation signal generating means is connected to the Y-directional wiring 63. The driving voltage applied to each electron-emitting device is supplied as a differential voltage between the scanning signal and modulation signal applied to the device.
The application of these electron-emitting devices to an image-forming apparatus necessitates emission currents with which phosphors emit light having sufficient brightness. On the other hand, it is also required that the electron-emitting devices are controlled so as to emit no electron under their OFF states. Also, needless to say, the increase of the number of steps of gradation is an important factor when image quality is enhanced. Further, to realize higher definition of a display, it is required that the diameter of an electron beam irradiated onto each phosphor is reduced and the number of pixels is also increased. It is also important that the electron-emitting devices are easy to manufacture.
An example of the conventional FE type electron-emitting device is a Spindt type electron-emitting device. The Spindt type electron-emitting device generally has a construction where a micro-tip is formed as an emission point and electrons are emitted from the tip thereof. With this construction, if an emission current density is increased to have a phosphor emit light, this causes the thermal destruction of an electron-emitting region, which limits the life span of the FE device. Also, the diameter of an electron beam emitted from the tip tends to be increased by an electric field formed by a gate electrode, which results in a shortcoming that it becomes impossible to decrease the beam diameter.
Various techniques have been proposed individually to overcome these shortcomings of the FE device.
There is proposed a technique such that, to prevent the increase of the electron beam diameter, a converging electrode is arranged over an electron-emitting region. In general, with this construction, the diameter of emitted electron beam is decreased by the negative potential of the converging electrode. However, the manufacturing process becomes complicated and therefore the manufacturing cost is increased.
With another technique, an electron beam diameter is decreased by eliminating a micro-tip like that used in the Spindt type electron-emitting device. Examples of this technique are described in JP 8-096703 A and JP 8-096704 A.
With this technique, electrons are emitted from a thin film arranged in a hole. In this case, a flat equipotential surface is formed on the surface of an electron-emitting film, so that there is obtained an advantage that the electron beam diameter is decreased. Also, by using a construction material having a low work function as an electron-emitting substance, electron emission becomes possible even without forming a micro-tip , which makes it possible to lower a driving voltage. There is also obtained an advantage that the manufacturing method is relatively simplified. Further, electron emission is performed in a plane area, so that electric fields do not excessively concentrate. As a result, the destruction of the tip does not occur and a long life span is realized.
In such an FE type electron-emitting device, an electric filed (1×108 V/m to 1×1010 V/m in usual cases of the Spindt type) that is necessary for electron emission is applied to an electron-emitting substance, which is usually connected to a cathode electrode, by a gate electrode arranged close to the electron-emitting substance. In this manner, it becomes possible to perform electron emission. Also, in usual cases, electrons emitted from an electron-emitting device are accelerated by an electric field formed between the device and an anode electrode arranged over the device. In this manner, there is given sufficient energy. The electrons that reach the anode electrode are captured by the anode electrode and are converted into an emission current.
In usual cases, modulation voltages applied between cathode electrodes and gate electrodes are set so as to fall within a range of from several tens of V to several hundreds of V, while voltages applied between the cathode electrodes and an anode electrode is set so as to fall within a range of from several hundreds of V to several tens of kV. That is, the voltages are increased by several ten to several hundred times as compared with the modulation voltages applied between the cathode electrodes and the gate electrodes.
Accordingly, ON-OFF control of electron emission from the devices is generally performed by modulating the voltages between the cathode electrodes and the gate electrodes to which small modulation voltages are applied. An example method of driving these electron-emitting devices is disclosed in JP 8-096703 A. This method is shown in FIG. 15 and will be briefly described below.
With the illustrated construction, anode voltages for RGB are modulated in a time-division manner to display a color image. Fundamentally, however, the voltage applied to an anode electrode is maintained constant (250 V) and a signal for image display is realized by modulating (20 V) the voltages applied between cathode electrodes and gate electrodes. Also, during an OFF period, both of the voltages of the cathode electrodes and the gate electrodes have the same potential and are set at 0V. Further, the distance between the cathode electrodes and the anode electrode is set at 300 μm. First, a potential of −β V is given to a cathode that is a selected scanning line and a potential of a V is applied to a gate that is a signal line for a required time period in accordance with the application of the potential of −β V. During this operation, a voltage of α+β V is applied between the gate and cathode, thereby emitting electrons. When one scanning period is finished, the potential of the cathode that is the selected scanning line becomes 0 V and the potential of a cathode that is the next selected scanning line becomes −β V, thereby repeating the operation described above in succession. Also, in the case where the anode potential is maintained constant, it is preferable that the distance between the cathodes and the anode is reduced to decrease a beam diameter. However, the indiscriminate reduction of the distance should be avoided in order to obtain a vacuum space without difficulty and to circumvent discharging.